A microlens has been used as an on-chip lens for solid-state imaging device such as a CCD (charge coupled device) or CMD (charge modulation device), or LCD (liquid crystal display) devices. In general, such microlens is formed by a following method. For example, as shown in FIG. 13A, a mask layer 20 composed of a resist film configured in a predetermined lens shape, is formed on a lens material layer 10 formed of an organic film composed of, e.g., an i-line photo-permeable resin. Further, by etching the mask layer 20 and the lens material layer 10, the lens shape of the mask layer 20 is transcribed to the lens material layer 10. Thus, a microlens 12 shown in FIG. 13B is formed.
In the solid-state imaging device employing the microlens, due to a miniaturization thereof, the pixel area becomes getting smaller, so that the amount of the incident light onto each pixel is getting smaller, deteriorating the sensitivity thereof. To this end, it is required to increase the area of the lens such that a larger amount of light is to be converged at the focal point. For the purpose, it is desirable to have a lens shape achieving a maximum lens area in each microlens, for example, a lens shape wherein a lens width of each microlens is increased while a distance A between adjacent lenses (see FIG. 13B) becomes decreased.
Accordingly, up to now, a microlens has been formed of a shape such that the distance A between adjacent lenses is as narrower as possible, by etching the mask layer 20 and the lens material layer 10 by using, e.g., CF4 gas as the processing gas (see, e.g., Japanese Patent Laid-open Publication Nos. H10-148704 and 2002-110952).
However, it takes very long time to employ the conventional method, which uses CF4 gas as the processing gas, to perform the etching process due to the low etching rate thereof. For this reason, there is a limitation in increasing the productivity of microlenses. Furthermore, since a longer etching time is required if the lens area in the microlens is increased and the distance A between adjacent lenses is decreased, it is impossible to enlarge the lens area while reducing the etching time in the conventional method.
Even in such conventional method, if the etching rate is the only concern, it is possible to enhance the etching rate by changing the parameters such as the flow rate of CF4 gas and the like during the etching process. However, if the parameters are changed during the etching process such that the etching rate is increased, the characteristics of the lens shape become degraded such that the resultant lens area of the microlens becomes small and the distance A between adjacent lenses increases.
Hereinafter, there will be described the results of the experiments wherein the parameters are changed during the etching process with reference to FIG. 14. FIG. 14 shows trends of changes in the etching rate and the distance between adjacent lenses when various parameters are changed during the etching process. The parameters related to the etching rate include a pressure in the processing chamber, a high frequency power applied to the electrode, a temperature of the mounting table, a flow rate of CF4 gas, and a flow rate ratio of CF4 gas to other gases (CHF3, CO, etc.) added thereto. In FIG. 14, as for the etching rate, for example, in case the arrow points to the acclivity, the etching rate tends to increase. Also, as for the distance A between lenses, for example, in case the arrow points to the acclivity, the lens width tends to decrease and the distance A between lenses tends to increase.
As can be seen from FIG. 14, when either the high frequency power or the flow rate of CF4 gas is increased, the etching rate tends to increase and the distance A between lenses also tends to increase. This is because, in case of using CF4 gas as the processing gas, it is hard to appropriately control to have a balance between F acting as an etching species of the mask layer 20 and the lens material layer 10 and C and the like acting as a deposit species, among the dissociation products generated while CF4 is plasmarized to be dissociated, even though the etching is performed by changing the parameters such as the flow rate of the processing gas and the pressure in the processing chamber.
As such, in the conventional method using CF4 gas as the processing gas, since there is a tradeoff between the etching rate and the lens area, it is impossible to increase the etching rate while enlarging the lens area at the same time.
Furthermore, there has been disclosed in Japanese Patent Laid-open Publication No. 2000-164837 a method for forming microlenses by using SF6 gas instead of CF4 gas as a processing gas. However, the processing gas in the above-described method contains O2 as well and the resultant lens area becomes small due to the small lens width. The reason for this is considered that the lens shape in the resist layer itself becomes smaller due to the fact that oxygen O facilitates the etching on the resist film, which is an organic film, among the dissociated products generated while the processing gas is plasmarized to be dissociated, resulting in a smaller lens shape in the lens material layer transcribed from the lens shape in the resist film. In the disclosures of Japanese Patent Laid-open Publication Nos. H10-148704 and 2002-110952, the processing gas also contains O2 gas, and therefore the resultant lens area becomes rather small as in Japanese Patent Laid-open Publication No. 2000-164837.